In recent years, various types of variable damping force dampers for vehicle having a damping force adjustable in a stepwise or continuous manner have been developed. Examples of known mechanisms of changing the damping force can include a mechanical mechanism of changing the area of an orifice in a piston using a rotary valve and an MRF-type mechanism of controlling the viscosity of a magneto-rheological fluid (hereinafter referred to as MRF) used in hydraulic oil by the use of a magnetic fluid valve on a piston. A vehicle equipped with such a variable damping force damper (hereinafter referred to simply as damper) can improve the steering stability and the riding comfort by variably controlling the damping force of the damper depending on the running state of the vehicle.
One known example of the method for improving the riding comfort is skyhook control based on the skyhook principle. The skyhook control of controlling the riding comfort (vibration damping control) sets a target damping force so as to suppress vertical movement of the sprung portion. Thus it is necessary to detect the sprung speed. Even when the damper has a characteristic in which the area of an orifice and the viscosity of an MRF are fixed, because the damping force varies in accordance with the stroke speed, it is necessary to detect the stroke speed, that is, the relative displacement speeds of the sprung and unsprung portions in order to perform the skyhook control.
Traditionally, a suspension control apparatus that performs the skyhook control needs to have a vertical G sensor and a stroke sensor for each wheel in order to detect the vertical sprung speed and stroke speed. However, because the stroke sensor is required to be attached to the inside of the wheel house or its adjacent area, it is difficult to find a sufficient space for accommodating it. To address this issue, a suspension control apparatus that includes no stroke sensor, calculates the relative displacement speeds of the sprung and unsprung portions from the amount of variation in the wheel speed, and controls the damping force of the damper on the basis of the calculated relative displacement speeds and others is proposed (see Japanese Unexamined Patent Application Publication No. 6-48139).
The suspension apparatus described in the above-mentioned patent literature calculates the relative displacement speeds of the sprung and unsprung portions by using the fact that the wheel speed varies as a result of relative displacement in a longitudinal (front and rear) direction occurring in accordance with the caster angle when the wheels relatively displace in a vertical direction with respect to the vehicle body by suspension geometry. Accordingly, if no caster angle is set in the suspension or it is very small, the calculation accuracy is lower or it is impossible to calculate the relative displacement speeds. Because the relative displacement speeds are calculated on the basis of the wheel speed, if the wheel slips, it is difficult to conduct precise control based on the relative displacement speeds, and the behavior of the vehicle may be unstable.